1. Field of the Invention
This invention relates generally to a battery management circuit, and more particularly, but not exclusively, to such a circuit capable of powering mobility vehicles.
2. State of the Art
Mobility vehicles, for instance medical mobility vehicles, which are currently on the market, typically use lead-acid batteries as their power source. Consequently the well established medical mobility vehicle industry uses system voltages, components and chargers that have been optimised to conform to this battery type.
Disadvantages with such systems are that the lead-acid batteries have a low energy to weight ratio compared to other types of battery that are available. It is desirable to make the battery as light as possible since in many instances the medical mobility vehicles, for instance wheelchairs and scooters, are designed to be dismantled and transported in motor vehicles to the place they will be used. There is therefore a trade off between the need to make the vehicle (consequently the battery) as light as possible and maximising the range over which the vehicle may be used.
Other battery types, for instance lithium-ion batteries, offer the potential for longer range and longer life cycles and are also lighter in weight, however they are not compatible with standard mobility vehicle components and chargers currently on the market. Further to this the nominal voltage delivered by a single lithium-ion polymer battery cell is 3.6 Volts and for a single lithium-ion phosphate cell 3.2V. Several lithium-ion battery cells would need to be stacked in series in order to provide sufficient voltage for many applications.
Rechargeable batteries are nearly all sensitive to the charging and discharging regime they experience over their operating lives. Lithium-ion cells are less sensitive to deep discharge cycles, however they have very strict maximum charging voltages and cells must be charged to their maximum voltage in order to maximise charge capacity. However, charging to a lower cell voltage and sacrificing some charge capacity enables the operating life of the cells to be significantly extended. This can be up to a factor of 2 or 4 times. Likewise, when lithium cells are discharged, there is a minimum allowable voltage across each cell and reducing the voltage beyond this limit can reduce the capacity and increase the cell self-discharge or leakage. By raising the minimum allowed cell voltage, the operating life of the cell or battery can be extended. This is especially the case where discharge currents are high.
In the case where the mobility vehicles are used very infrequently, the vehicle and battery may be stored for long periods. It is undesirable to store the battery with cell voltages at or near the maximum and minimum allowed values since the battery or individual cells will deteriorate and potentially lose charge storage capacity or exhibit increased self-discharge. Furthermore, storing cells with cell voltages at their maximum increases the risk of cell failure and fire.